While various individual methods are available for addressing specific constituents of gas well wastewaters and for treatment of abandoned coal mine acid drainage (AMD), there exists no process to treat, or co-treat, such wastewaters in a manner that renders such wastewaters suitable for recycle.
The drilling of natural gas wells and subsequent on-going recovery and/or production of natural gas produces a number of wastewater streams commonly identified as completion, hydrofracture flowback, and production waters. Drilling of a gas well also requires a substantial amount of water for makeup of the virgin, prior to use, drilling muds, completion, and hydrofracture waters. It is estimated that a typical deep horizontal gas well requires from 1 to 3 million gallons of water for completion/hydrofracture and generates an equal amount of hydrofracture flowback and production wastewaters.
With significant new large drilling activities linked to tight gas shale formations such as the Marcellus Shale in Pennsylvania, the provision of sufficient water for new drilling activities and subsequent disposal of large volumes of wastewater has become a critical issue. The wastewaters produced by gas well drilling, completion, and production activities present some unusual and difficult problems with regard to treatment suitable to enable disposal by discharge to surface waters.
Recent disposal activities have included co-treatment in publicly owned treatment plants (POTW), the use of industrial treatment systems, and the use of a limited number of purpose built treatment plants. These methods of disposal all treat and discharge treated wastewater to surface waters but are capable of removing only a limited number and amount of the pollutants typically present. POTW are limited in that the amount of wastewater that can be treated is limited by the barium content, which can affect the production of a sludge characterized as a hazardous waste. Other processes are limited in that they cannot obtain a substantial reduction in dissolved solids.
In 2008, the Monongahela River in Western Pennsylvania experienced a rapid rise in dissolved solids content which severely disrupted many public water supplies and industrial operations. The cause was found to be the discharge of gas well wastewaters treated by many POTW situated along the river.
The regulatory agency, Pennsylvania Department of Environmental Protection, (PADEP) subsequently placed a very restrictive limit of 500 mg/l on dissolved solids discharges to surface waters resulting from treatment of gas well wastewaters. This limit goes into effect on Jan. 1, 2011, for all new dischargers with a starting discharge date of Apr. 11, 2009.
Table 1 below shows analytical data on a typical hydrofracture flowback wastewater. As can be seen, it is extremely high in dissolved solids, toxic barium, and scale formers such as calcium, iron, magnesium, and strontium.
TABLE 1ParameterAnalytical Resultconductivity - mmhos152,000dissolved solids - mg/l175,268total suspended solids - mg/l416biological oxygen demand, 5 day - mg/l489methyl blue active substances - mg/l0.939chloride - mg/l73,370oil/grease - mg/l38total organic carbon - mg/l114.5ammonia-N - mg/l83.5chemical oxygen demand - mg/l600total hardness - mg/l39,100strontium - mg/l6,830barium - mg/l3,310calcium - mg/l14,100iron - mg/l52.5magnesium - mg/l938manganese - mg/l5.17
In addition to these major constituents, low levels of copper, nickel, zinc, lead, and other assorted heavy metals are also reported.
The only known technology for treatment of such a wastewater to meet the impending PADEP dissolved solids limit for surface water discharge is evaporation. Prior to evaporation, the toxic barium would have to be removed to prevent the resultant dry salt cake from being a hazardous waste while the scale formers calcium, magnesium, iron, and strontium would have to be removed to prevent scale formation on heat transfer surfaces.
An alternative to trying to treat for surface water discharge, or pretreat and evaporate, would be to treat the wastewater for recycle as hydrofracture makeup water. From various sources in the gas well hydrofracture service industry, it appears that the specific parameters shown in Table 2 below would be required of water to be used for makeup of hydrofracture water.
TABLE 2ParameterRecommended ValuesScale Ions - aluminum, barium,maximum of 2,500 mg/l as CaCO3calcium, iron, magnesium,manganese, and strontiumDissolved Solidsmaximum of 50,000 mg/lIronmaximum of 20 mg/lSuspended SolidsnoneCalciummaximum 350 mg/l as CaCO3
The makeup water would need to be substantially free of microorganisms to prevent growth of microorganisms in the fractured gas bearing strata.